There are numerous situations where it is desirable to design a product which contains a metal substrate and is resistant to fire. For instance, fire performance cables are required to continue to operate and provide circuit integrity when they are subjected to fire. To meet some of the standards, cables must typically maintain electrical circuit integrity when heated to a specified temperature (e.g. 650, 750, 950, 1050° C.) in a prescribed way for a specified time (e.g. 15 minutes, 30 minutes, 60 minutes, 2 hours). In some cases the cables are subjected to regular mechanical shocks, before, during and after the heating stage. Often they are also subjected to water jet or spray, either in the latter stages of the heating cycle or after the heating stage in order to gage their performance against other factors likely to be experienced during a fire.
These requirements for fire performance cables have been met previously by wrapping the conductor of the cable with tape made with glass fibres and treated with mica. Such tapes are wrapped around the conductor during production and then at least one insulative layer is subsequently applied. Upon being exposed to increasing temperatures, the outer insulative layers are degraded and fall away, but the glass fibres hold the mica in place. These tapes have been found to be effective for maintaining circuit integrity in fires, but because of the additional manufacturing steps they are quite expensive to produce. Further the process of wrapping the tape around the cable is relatively slow compared to other cable production steps and thus, wrapping the tape slows overall production of the cable further adding to the costs. Attempts have been made to reduce the costs by avoiding the use of tape and extruding a cable coating consisting of a flexible polymeric composition which forms an insulating ceramic when exposed to fire to provide the continuing circuit integrity.
Such ceramic forming compositions are known in the prior art. For example, U.S. Pat. No. 4,269,753 and U.S. Pat. No. 4,269,757 describe coatings of ceramic forming compositions being applied directly to a short length of copper wire. When the coated wire is exposed for 30 minutes to air, at 850° C., the coatings are said to form a strong and hard ceramic substance without any cracks and without separating from the copper wire. U.S. Pat. No. 6,387,512 shows application of a ceramic forming coating to an electrical conductor and the retention of circuit integrity when this is heated for 2 hours at 930° C. with an applied potential of 500 volts. International Application No. PCT/AU2003/00968 in the name of Polymers Australia Pty Ltd discloses a silicone polymer based ceramic forming composition suitable for cables and other applications which forms a self supporting ceramic material when heated to an elevated temperature. International Application No. PCT/AU2003/01383 also in the name of Polymers Australia Pty Ltd discloses a self supporting ceramic forming composition suitable for cables and other applications which exhibit little, or no shrinkage, when exposed to the kind of elevated temperatures associated with a fire.
While the ceramic forming compositions of the prior art, in theory, are able to provide the required electrical and/or thermal insulation, the other physical properties of ceramic forming compositions, both before and after exposure to elevated temperatures, make the practical application of these materials, particularly in cable applications, difficult to implement with compromises needing to be made to accommodate the less than ideal physical properties. Ideally the ceramic forming layer should be able to accommodate the mismatch between the thermal coefficients of expansion of the metal substrate and the ceramic forming composition during the increasing temperatures experienced during a fire and the decreasing temperatures after the fire, have adequate mechanical properties before, during and after exposure to elevated temperatures, maintain its structural integrity and where necessary provide an adequate water barrier, particularly during and after exposure to elevated temperatures.
Hence it is an object of the invention to provide a fire performance cable or fire performance article from a ceramic forming material on a metal substrate which overcomes one or more of the practical problems associated with using ceramic forming materials.